Optical Coherence Tomography (OCT) is a technique for obtaining sub-surface images of translucent or opaque materials at a resolution equivalent to a low-power microscope. It is effectively ‘optical ultrasound’, imaging reflections from within tissue to provide cross-sectional images. It is known to use Optical Coherence Tomography (OCT) for taking cross-sectional pictures of the retina in order to diagnose and follow treatment in certain eye conditions and diseases.
Light in an OCT system is broken into two arms—a sample arm (containing the item of interest) and a reference arm (usually a mirror). The combination of reflected light from the sample arm and reference light from the reference arm gives rise to an interference pattern, but only if light from both arms have an optical difference of less than a coherence length. By scanning the mirror in the reference arm, a reflectivity profile of the sample can be obtained (this is time domain OCT). Areas of the sample that reflect back a lot of light will create greater interference than areas that do not. Any light that is outside the short coherence length will not interfere. This reflectivity profile, called an A-scan, contains information about the spatial dimensions and location of structures within the item of interest. A cross-sectional tomograph (B-scan) may be achieved by laterally combining a series of these axial depth scans (A-scan).
WO 2006/103663 discloses a method and apparatus for analyzing optical properties of an object using a light beam having a plurality of amplitudes, phases and polarizations of a plurality of wavelengths impinging from the object, obtaining modified illuminations corresponding to the light beam, modulating the light beam, analyzing the modulated light beam, and obtaining a plurality of amplitudes, phases and polarizations maps of the plurality of wavelengths, which are used to determine the object's optical properties.
The use of OCT for modulating the light before it strikes the object is described, for example, in WO 2008/087613, which discloses an apparatus and method combining achromatic complex Fourier domain OCT signal reconstruction with a common path and dual beam configuration. Light is directed through an interferometer, which splits the light to a dual beam and directs the two beams to the object in a common path. The combined dual beam interacts with a multi-layered object to obtain tomograms of a single point of the object. A B scan is required.
U.S. Pat. No. 7,281,801 describes a system and method for measuring the thickness of a tear film layer and the heights of tear menisci around upper and lower eyelids of an eye. A plurality of images are acquired between consecutive blinks the eye using optical coherence tomography (OCT). The images depict the tear film layer and tear menisci as distinct from the cornea of the eye. In an embodiment, a plurality of reflectivity profiles from an OCT image are aligned and averaged. The difference between a first peak and a second peak of the average reflectivity profile is measured to determine the thickness of the tear film layer. A B scan is required.
Alex Zlotnik et al. “Full Field Spectral Domain Optical Coherence Tomography with Improved Extended Depth of Focus”, OSA/CLEO/QELS 2010 discloses the use of an extended light source by creating interference fringes at the focal plane of a lens. An interfering phase mask is used to extend the depth of focus.
Drexler et al. “Dual Beam Optical Coherence Tomography” in Signal Identification for Ophthalmologic Diagnosis directs reference and object beams to the object. The light that is reflected back by the cornea serves as a reference for the light reflected by the retina. This requires a B scan and requires a sensor having very high spectral resolution owing to the high optical path difference between the reference beam and the object beam.
WO/2008/087613 discloses an apparatus and a method combining achromatic complex Fourier Domain OCT signal reconstruction with a common path and dual beam configuration. The apparatus directs a modulated interferometric point light source to an object to be measured and is not able to measure optical characteristics of a two-dimensional object within an optical system other than by point-by-point scanning.
US 2009/0080739 discloses a similar approach for performing spectral OCT imaging on a target by repeatedly scanning the target along a transverse scanning line with an object beam derived from an OCT interferometer having a narrowband source. The wavelength of the narrowband source is modulated over a range of wavelengths at a rate that is slow relative to the rate of scanning the target. The object beam returned from the target is detected to produce a set of data obtained from multiple scans along said scanning line over the entire range of wavelengths. The data is then processed to extract an OCT image (typically a B-scan) of the target containing depth information.
The above-referenced publications are representative of those that use OCT to image successive points of an object and thus require scanning of the OCT beam over a complete area of interest. OCT systems of this type involve the use of short coherent light, that is, light with a distinct spectral width and therefore short time coherence. The object is scanned point by point along a line extending on the object surface in the x-direction by the measurement beam of an interferometer. Under every surface point the measurement beam also penetrates into the object (in the z-direction) and the diffusely reflected light is interfered with the reference beam of the interferometer. Interference occurs because of the use of short coherence light only when the measurement beam and reference beam have the same path length within the coherence length.
The literature recognizes the deficiency of such an approach and addresses the need to perform area imaging. Thus, for example, U.S. Pat. No. 7,695,140 (Fercher) describes an ophthalmologic measuring method that can depict three-dimensional structures of the interfaces of an eye by means of low coherence interferometry based on reference points. To this end, the pupil is illuminated at a number of points by a low coherence light source. The measurement radiation reflected at these points by the interfaces and surfaces of the eye is superimposed with a reference radiation. The measurement data generated thereby are spectrally split up by a diffraction grating, projected onto a two-dimensional detector array, and routed to a control unit that determines a three-dimensional structure of all intraocular interfaces and surfaces of the eye. This makes it possible to determine the depth positions of the measuring beams at many pupil points with a single image taken by the array camera by illuminating the pupil with an aperture grid, and the reference mirror contains a periodic phase grid.
U.S. Pat. No. 6,810,140 discloses a system for three dimensional real-time imaging apparatus of the ocular retina, wherein laser rays are formed into a two dimensional ray surface sequentially with time by using a polygon mirror motor and galvanometer and irradiated on the almost transparent retina through the pupil. The optical system is complex and the polygon mirror performs optical scanning.
There is thus required a method and system that uses a dual beam and produces a two-dimensional area image without the need for scanning.